1. Field of the Invention
The present invention relates to an image recording apparatus used in a copy machine or printer.
2. Description of the Related Art
An error diffusion method or error distribution method is known as a method of processing an input image signal to record a halftone image. The error diffusion method is a technique for performing binary coding process of an image signal so as to minimize a density difference between an input image signal and an output image signal. This technique is very effective to output both a halftone image and a high-definition image in a printer having only binary representation. A method of inputting a signal processed by this error diffusion method to a printer and recording the processed signal at the printer is called error diffusion recording. This error diffusion recording has a disadvantage in that a texture inherent to a smooth halftone image becomes conspicuous.
To compensate for the drawback of this error diffusion method, a multilevel error diffusion method and a multilevel correlative density assignment of adjacent pixels ("multilevel algorithm for high quality pseudo halftoning", Proceedings of Television, Vol. 44, No. 5, pp. 608-614 (1990)) which is similar to the multilevel error diffusion method are known. In these techniques, however, a tone gradation tends to be conspicuous in a highlighted or high-density portion whose recording is unstable. A pseudo contour is formed in such a portion to result in a poor image.
To solve this problem, a method of adding random noise to an image signal (e.g., "Smoothing Effect in Gray Scale Characteristics Using a Pseudo-Noise Method", Proceedings of Television, Vol. 40, No. 11, p. 1113 (1986)). According to this method, however, the texture of a multilevel recording portion becomes different from that of the portion added with random noise, resulting in a poor image.
As a method of correcting a tone gradation, a technique for correcting the levels of a signal to smooth levels in advance, using a ROM table is known. In this tone correction, however, when a change in density occurs in a printer deterioration over time, the pseudo contour tends to be formed in the highlighted or high-density portion.
In error diffusion recording, when an input image signal is subjected to undercolor removal, snow noise tends to be conspicuous. Published Unexamined Japanese Patent Application No. 3-204273 describes a technique for determining a condition in selecting each color to prevent colors from generating at random, thereby preventing snow noise. In a practical color printer, however, color misregistration between colors to be printed occurs to result in a random color combination in stacking the colors to actually print an image under a specific condition for a color selection. Snow noise is not always reduced. When four-color printing is performed after processing such as undercolor removal, a low-density recording signal is frequently generated to form a number of non-recorded dots in a system in which low-density recording is unstable. As a result, color reproduction is often degraded.
In a color copy machine or color printer, a positional error occurs between ink colors in a color printing unit. Black characters are blurred, and the color of a halftone portion such as a gray or skin-colored portion changes depending on different positions within the printed paper sheet or different paper sheets. To prevent this, a technique for using a screen angle in color halftone reproduction to obtain different screen angles for all colors, locally causing an average positional error any time, and stabilizing color reproduction is known. According to this technique, image degradation in a character portion greatly occurs, and moire noise is undesirably generated.
To improve the quality of a character portion, Published Unexamined Japanese Patent Application No. 63-240175 discloses a technique for extracting a character and black regions of a color original, performing 100% undercolor removal of only a black character region, and performing undercolor removal of less than 100% of the remaining region. In practice, however, it is difficult to accurately extract the character region in 100%. When the character region is erroneously identified, e.g., when a halftone image region is erroneously identified as the character region, 100% undercolor removal causes a change in color or an increase in snow noise. It is difficult to set an extreme undercolor removal ratio (i.e., blackening ratio). A color variation caused by a relative positional error in an ink color of the printer occurs in the halftone image region, and moire noise cannot be reduced. When undercolor removal of a region (i.e., a halftone image region occupying an almost print area) except for the character region is performed at a low ratio, an ink consumption amount cannot be reduced. In addition, every time the undercolor removal ratio changes, the color changes in strict color reproduction. It is, therefore, difficult to variably set the undercolor removal ratio.
As described above, in the conventional error diffusion recording technique, texture noise inherent to binary error diffusion recording is generated, and pseudo contour noise at discontinuous points in multilevel recording is generated. Even if processing for correcting printer tone characteristics is performed, the pseudo contour caused by variations in tone characteristics due to the deterioration over time is undesirably formed.
In the conventional color image recording apparatus, when four-color printing is performed in error diffusion recording, snow noise caused by color noise generated at random is generated. In addition, it is difficult to faithfully reproduce the colors in a printer having unstable tone characteristics, and color reproduction by undercolor removal processing is degraded.
In the conventional color image recording apparatus, a character portion tends to be blurred. When the technique having different undercolor removal ratios in the character and halftone image regions is used, a change in color in the character region occurs and snow noise increases due to erroneous identification of the region. In addition, the ink consumption amount cannot be reduced.